U.S. patent application number 15/359617 was filed with the patent office on 2017-06-22 for silica aerogel, heat-insulation material, and method for producing silica aerogel.
The applicant listed for this patent is Panasonic Intellectual Property Management Co., Ltd.. Invention is credited to KAZUMA OIKAWA, SHIGEAKI SAKATANI, KEI TOYOTA.
Application Number | 20170174859 15/359617 |
Document ID | / |
Family ID | 58994184 |
Filed Date | 2017-06-22 |
United States Patent
Application |
20170174859 |
Kind Code |
A1 |
SAKATANI; SHIGEAKI ; et
al. |
June 22, 2017 |
SILICA AEROGEL, HEAT-INSULATION MATERIAL, AND METHOD FOR PRODUCING
SILICA AEROGEL
Abstract
A method for producing a silica aerogel, includes: (i) adding an
electroconductive polymer to the sol of an aqueous alkaline
silicate solution to convert the sol to a gel; (ii) aging the gel
to cause said gel to grow; (iii) hydrophobizing the gel; and (iv)
drying the gel. Further provided is a silica aerogel including an
electroconductive polymer. Still further provided is a
heat-insulation material, including the above-described silica
aerogel and fibers.
Inventors: |
SAKATANI; SHIGEAKI; (Osaka,
JP) ; OIKAWA; KAZUMA; (Osaka, JP) ; TOYOTA;
KEI; (Osaka, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Panasonic Intellectual Property Management Co., Ltd. |
Osaka |
|
JP |
|
|
Family ID: |
58994184 |
Appl. No.: |
15/359617 |
Filed: |
November 22, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F16L 59/028 20130101;
C08J 9/28 20130101; C08J 9/0066 20130101; C08J 2201/05 20130101;
C01B 33/155 20130101; C08J 2205/026 20130101; C08J 2365/00
20130101 |
International
Class: |
C08J 9/28 20060101
C08J009/28; C08J 9/00 20060101 C08J009/00; F16L 59/02 20060101
F16L059/02; C01B 33/155 20060101 C01B033/155 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 17, 2015 |
JP |
2015-245848 |
Claims
1. A method for producing a silica aerogel, comprising: (i) adding
an electroconductive polymer to a sol of an aqueous alkaline
silicate solution to convert the sol to a gel; (ii) aging the gel
to cause said gel to grow; (iii) hydrophobizing the gel; and (iv)
drying the gel.
2. The method for producing a silica aerogel according to claim 1,
wherein the electroconductive polymer is water-soluble.
3. The method for producing a silica aerogel according to claim 1,
wherein the electroconductive polymer includes any of the
followings: (3-thiophene-ethylsulfonic acid); a polythiophene
derivative of 3,4-ethylenedioxythiophene; polyaniline; and a
copolymer of ethyl 3-methyl-4-pyrrolecarboxylate and butyl
3-methyl-4-pyrrolecarboxylate.
4. The method for producing a silica aerogel according to claim 1,
wherein a water-soluble polymer is further added to the sol in Step
(i).
5. The method for producing a silica aerogel according to claim 1,
wherein the aqueous alkaline silicate solution is a solution that
is synthesized from a water dispersion or an aqueous solution of
silica fine particles.
6. The method for producing a silica aerogel according to claim 5,
wherein the electroconductive polymer is in the form of particles
having a particle size of 1 to 100 nm, and a solution in which the
particles of the electroconductive polymer are dispersed is added
to an aqueous sol solution using water glass as a starting material
to synthesize the gel.
7. The method for producing a silica aerogel according to claim 1,
wherein, in step (ii), the gel is allowed to stand at 50.degree. C.
to 100.degree. C. under ordinary pressure for 6 to 18 hours.
8. A silica aerogel comprising an electroconductive polymer.
9. The silica aerogel according to claim 8, further comprising a
water-soluble polymer.
10. The silica aerogel according to claim 8, wherein the
water-soluble polymer and the electroconductive polymer form a
copolymer.
11. The silica aerogel according to claim 9, wherein the
water-soluble polymer intramolecularly has at least one of the
following functional groups; an amino group; a hydroxyl group; a
carboxyl group; a carbonyl group; and a sulfo group.
12. The silica aerogel according to claim 8, wherein the
electroconductive polymer includes at least one of the followings:
a polypyrrole; a polythiophene; and a polyaniline.
13. The silica aerogel according to claim 11, wherein the
electroconductive polymer includes any of the followings:
poly(3-thiophene-ethylsulfonic acid); a polythiophene derivative of
3,4-ethylenedioxythiophene; polyaniline; and a copolymer of ethyl
3-methyl-4-pyrrolecarboxylate and butyl
3-methyl-4-pyrrolecarboxylate.
14. The silica aerogel according to claim 8, wherein the
electroconductive polymer is in the form of particles having a
particle size of 1 to 100 nm.
15. A heat-insulation material, comprising: the silica aerogel
according to claim 8; and fibers.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a silica aerogel, a
heat-insulation material, and a method for producing a silica
aerogel.
BACKGROUND
[0002] Silica aerogels differs from urethane foams (PU), foamed
polystyrene (EPS), and vacuum insulation panels (VIPs), in that
there are almost no changes in their heat-insulation performance
across the ages. Furthermore, silica aerogels have heat resistance
of up to about 400.degree. C. For these reasons, silica aerogels
have attracted a great deal of attention as next-generation
heat-insulation materials.
[0003] With regard to PUs and EPS that are obtained through foaming
with a gas having a low heat conductivity, their heat-insulation
performance deteriorates as the gas comes out of the materials over
time. Moreover, PUs and EPS have poor heat resistance. VIPs have
excellent heat-insulation efficiencies of several milliWatts per
milliKelvin. However, over time, trace amounts of molecules of air
penetrate into VIPs from their portions that have been bonded
through thermal fusion bonding when core materials are
vacuum-encapsulated, resulting in loss of vacuum, and therefore
causing problem of degradation across the ages. Furthermore, there
is also a problem that VIPs have a heat resistance of only about
100.degree. C.
[0004] Silica aerogels are superior to any other existing
heat-insulation materials in terms of deterioration with age and
heat resistance. Silica aerogels have excellent heat conductivities
of around 15 mW/mK. However, silica aerogels have network
structures in which silica particles on the scale of several tens
of nanometers are connected in rows through point contact.
Accordingly, silica aerogels do not have sufficient mechanical
strength. Therefore, in order to overcome this weakness, studies
have been made to improve the strength by way of combining silica
aerogels with fibers, unwoven fabrics, resins, etc.
[0005] In general, inorganic nanoporous materials such as silica
aerogels are synthesized by the sol-gel method, which is a
liquid-phase reaction. Water glass (an aqueous solution of sodium
silicate) or alkoxysilane compounds such as tetramethoxysilane are
used as raw materials. These materials, and a liquid medium such as
water or alcohols, and, as needed, a catalyst are mixed, and are
hydrolyzed. That is, the materials are subjected to
polycondensation in a liquid medium to thus form a wet gel. Then,
the wet gel is subjected to a silylation reaction. Finally, the
liquid medium inside the wet gel is evaporated to dry the gel.
Synthesis of inorganic nanoporous materials are described in
WO/2007/010949, JP-A-7-257918, and JP-A-2003-183529.
SUMMARY
[0006] However, strength of aerogels synthesized by conventional
arts are low in terms of their structures, and the aerogels are
predisposed to charge since they have high electrical insulation
properties. For example, when silica aerogels are formed into thin
films, e.g., films with a thickness of 100 .mu.m, there is a
problem that the films are difficult to handle due to static
electricity. Furthermore, silica aerogels also have a problem in
which powder falling frequently occurs, and charged powders are
transferred to adjacent areas.
[0007] As one technique for preventing their electrical charging, a
technique in which addition of an electroconductive material such
as carbon is involved can be mentioned. However, in techniques
including addition of such a material, "transparency," which is one
of features of aerogels, will be impaired.
[0008] Moreover, inclusion of a hydrophilic polymer in the raw
materials can be considered in order to prevent electrical
charging. However, such a polymer is not compatible with an aqueous
sol solution based on water glass that serves as one raw material,
and it becomes impossible to synthesize an aerogel. Furthermore,
even if the aqueous sol solution turns into a gel, the resulting
gel will be turbid, and the transparency will be impaired.
[0009] Additionally, static elimination using an ionizer, or
reforming based on plasma discharging can be applied to silica
aerogels. However, these techniques merely bring about temporal
effects, and any persistent effects of electrical charging cannot
be expected.
[0010] Thus, it has been impossible to prevent electrical charging
while maintaining transparency in silica aerogels.
[0011] Therefore, purposes of the disclosure are to provide a
silica aerogel in which the charge amount is reduced while
transparency intrinsically possessed by silica aerogels is not
impaired, and to further provide a heat-insulation material using
the silica aerogel, and a method for producing the silica
aerogel.
[0012] As solutions to achieve the above-described purposes,
provided is a method for producing a silica aerogel, including: (i)
adding an electroconductive polymer to a sol of an aqueous alkaline
silicate solution to convert the sol to a gel; (ii) aging the gel
to cause said gel to grow; and (iii) hydrophobizing the gel; and
(iv) drying the gel. Further provided is a silica aerogel including
an electroconductive polymer. Still further provided is a
heat-insulation material, including: the above-described silica
aerogel; and fibers.
[0013] According to the disclosure, while excellent heat-insulation
performance, transparency, and heat resistance of silica aerogel
are maintained, the heat-insulation resistance is reduced, and
thus, it becomes possible to synthesize heat-insulation particles
and a thin heat-insulation sheet in which contamination due to
powder falling, or deficiency in handling properties due to
electrostatic charging will not occur.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a diagram that shows one example of steps for
producing a silica aerogel in an embodiment.
[0015] FIGS. 2A-2C are structural diagrams that show examples of an
electroconductive polymer in an embodiment.
[0016] FIG. 3 is a structural diagram that shows an example of an
electroconductive polymer in an embodiment.
[0017] FIG. 4 is a structural diagram that shows an example of a
water-soluble electroconductive polymer in an embodiment.
[0018] FIG. 5 is a lateral view of a screw tube that is used for
evaluation in an embodiment.
[0019] FIG. 6 shows a cross-section view of a heat-insulation
material according to one embodiment.
DESCRIPTION OF EMBODIMENTS
[0020] One embodiment of the disclosure will be described with
reference to FIG. 1.
[0021] A method for producing a silica aerogel according to an
embodiment is characterized in that a water-soluble or
water-dispersible electroconductive polymer is added to a sol
solution. This makes it possible to prepare an aerogel that hardly
electrically charges while having sufficient insulation
properties.
[0022] In addition, a method for synthesizing an aerogel in this
embodiment can be incorporated also into any other synthesis
methods using water glass-based materials. Conditions described
herein are one example, and adoptable conditions are not limited to
the described conditions.
(Overview of the Production Method)
[0023] The silica aerogel and the method for producing the same
according to this embodiment will now be described. Steps for
producing the aerogel according to this embodiment will be shown
below.
[0024] One example of the states in the respective steps from
preparation to drying of a sol 101 are shown in FIG. 1.
[0025] The gel-preparation step (1) is a step in which an aqueous
alkaline silicate solution is converted into a gel. At first, an
aqueous alkaline high-molar-ratio silicate solution with a silicate
concentration of 10% to 20% is prepared from an aqueous alkaline
low-molar-ratio silicate solution with a molar ratio
SiO.sub.2/Na.sub.2O of about 0.5 to 4. Then, hydrochloric acid and
an electroconductive polymer are added to the
high-molar-ratio-silicate solution, and the resulting mixture is
stirred to adjust the pH to 7.0 to 7.5, thus converting the sol 101
into a gel.
[0026] The aging step (2) is a step for reinforcing a skeleton of
silica after gelatinization. A hydrogel 102 is heated for 12 hours
in a furnace at 80.degree. C. This reinforces the skeleton of the
hydrogel, and thus, a hydrogel 103 with a reinforced skeleton is
prepared.
[0027] The hydrophobization step (3) is a step for hydrophobizing a
surface of the aerogel in order to prevent contraction of the
aerogel during drying. The hydrogel is caused to react with an
active species (trimethylsilyl chloride in this embodiment) in a
mixture solution of hexamethyldisiloxane (HMDSO), hydrochloric
acid, and 2-propanol, in a furnace at 55.degree. C. for 12 hours,
thereby preparing a surface-modified gel 104.
[0028] In the drying step (4), the gel with a modified surface
(modified with a trimethylsilyl group in this embodiment) is dried
in a furnace at 150.degree. C. for 2 hours, thereby preparing an
aerogel 105.
[0029] In addition, for a value of insulation resistance (ohm) of
the resulting gel, a value on the scale of 10.sup.6-11 is
preferable. If the value is on the scale of 10.sup.12 or more, the
insulation properties will be too high, and therefore, a state in
which the gel is likely to bear static electricity will be
maintained. On the other hand, if the value is on the scale of
10.sup.5 or less, a possibly-separated powder of the gel or
electroconductive polymer needs to be regarded as conductive, and
therefore, such a scale is not preferable for electric insulation
sheets.
Details on Production
(1) Gel-Preparation Step
[0030] In the gel-production step, an acid is added to a basic
aqueous high-molar-ratio-silicate solution to make the solution
acidic, and then, polycondensation is carried out.
[0031] The aqueous alkaline high-molar-ratio-silicate solution is
produced from water glass. Water glass is an aqueous sodium
silicate solution or an aqueous silicate soda solution, and is a
liquid in which SiO.sub.2 (silica) and Na.sub.2O (sodium oxide) are
dissolved in H.sub.2O at various ratios.
[0032] A molecular formula of water glass is Na.sub.2O nSiO.sub.2
mH.sub.2O in which n is a molar ratio representing a mixing ratio
of Na.sub.2O and SiO.sub.2. The aqueous high-molar-ratio-silicate
solution is a material that is obtained by removing sodium, which
is unnecessary for formation of aerogels, from water glass,
followed by stabilization of the material at the basic region, and
is neither water glass nor colloidal silica. As one of features of
the aqueous high-molar-ratio-silicate solution, it can be mentioned
that a particle diameter of the sol falls within an
intermediate-size range (1-10 nm) between sizes of water glass and
colloidal silica.
[0033] If an aqueous silicate solution with a sol particle diameter
of less than 1 nm is used, simultaneous pursuit of the
above-mentioned small pore diameter and specific surface area
becomes difficult. Therefore, consequently, only fragile and
breakable aerogels are synthesized.
[0034] If silica with a sol particle diameter of more than 10 nm is
used, the reactivity is lowered, and therefore, a homogenous gel
cannot be formed.
<Aqueous High-Molar-Ratio-Silicate Solution>
[0035] With regard to a method for producing the aqueous
high-molar-ratio-silicate solution, the aqueous
high-molar-ratio-silicate solution can be produced at least by the
following steps using an aqueous alkaline low-molar-ratio silicate
solution as a starting material.
[0036] (a) adding an acid to an aqueous alkaline low-molar-ratio
silicate solution to produce a by-product salt;
[0037] (b) immediately after above Step (a), bringing the aqueous
solution into contact with a pressure-driving semipermeable
membrane to concentrate the aqueous solution, and simultaneously
separating and removing the by-product salt produced in above Step
(a); and
[0038] (C) subsequent to above Step (b), or simultaneously with
above Step (b), continuously or intermittently adding water to the
aqueous solution, and again bringing the aqueous solution into
contact with the pressure-driving semipermeable membrane in above
Step (b) to concentrate the aqueous solution, and simultaneously
separating and removing the by-product salt produced in above Step
(a), in a repetitive manner. The aqueous high-molar-ratio-silicate
solution in this embodiment is an aqueous silicate solution
including 10% or more of silica.
[0039] As a method for producing an aqueous silicate solution
including 10% or more of silica, although the above-described
method is adopted in this embodiment, it is not limited to that
method. However, when a general aqueous water glass solution No. 4
is caused to pass through an ion-exchange resin to remove sodium,
salts are deposited on the surface of the ion-exchange resin, and
therefore, removal of sodium cannot efficiently be carried out,
unless the aqueous solution is diluted to less than 10%.
Accordingly, in this technical field, when sodium is removed from
water glass that is an aqueous alkaline low-molar-ratio-silicate
solution, generally, the aqueous solution is diluted to less than
10%, and then, is converted into a gel through a
dehydration-condensation reaction. Therefore, according to such a
technique, it is difficult to increase the concentration of silica
to the concentration level achieved by the present embodiment.
[0040] The molar ratio of the aqueous high-molar-ratio-silicate
solution which is used as a material in this embodiment is
preferably 15 to 30, and is more preferably 20 to 30 in order to
reduce the aging time or to improve the strength of the gel
skeleton.
[0041] The aqueous high-molar-ratio-silicate solution that is used
as a material in this embodiment preferably have a silicate
concentration of 10% to 20%, more preferably 12% to 16%.
[0042] If the silicate concentration is less than 10%, the strength
of the skeleton of the wet gel may be insufficient in the same
manner as conventional arts, since the silicate concentration is
low.
[0043] If the silicate concentration exceeds 20%, a time required
for gelatinization of the sol solution is rapidly shortened, and it
may be impossible to control the gelatinizing time.
<Electroconductive Polymer>
[0044] When the sol is converted into a gel, an electroconductive
polymer is added to the reaction mixture. For the electroconductive
polymer, polypyrroles as shown in FIG. 2A, polythiophenes as shown
in FIG. 2B, and polyanilines as shown in FIG. 2C can be used. In
this embodiment, a water-soluble or water-dispersible
electroconductive polymer is preferably used as the
electroconductive polymer.
[0045] For the electroconductive polymer, for example, as shown in
FIG. 3, a molecule having a structure including
poly(3-thiophene-ethylsulfonic acid), which is water-soluble and
which can be obtained by introducing a substituent group directly
into a monomer, can be used.
<Copolymer of a Water-Soluble Polymer and an Electroconductive
Polymer>
[0046] Also, a water-soluble polymer intramolecularly having a
sulfo group, which is compatible with water, can be used as a
dopant/dispersing agent.
[0047] That is, monomers that constitute an electroconductive
polymer are oxidatively polymerized in an aqueous solution of a
water-soluble polymer. According to this process, a part of sulfo
groups possessed by the water-soluble polymer is doped to the
electroconductive polymer. Furthermore, the water-soluble polymer
and the electroconductive polymer are integrated to form a
water-soluble electroconductive polymer.
[0048] As a result, due to the rest of sulfo groups, water
solubility can be imparted to the copolymer of the
electroconductive polymer and the water-soluble polymer.
Accordingly, a water solution in which electroconductive polymers
are finely dispersed can be prepared.
[0049] In this case, the water-soluble polymer intramolecularly has
at least one highly-polar functional group (e.g. an amino group,
hydroxyl group, carboxyl group, carbonyl group, and sulfo group).
Accordingly, the water-soluble polymer is easily mixed with water
and the electroconductive polymer, and the reaction homogenously
proceeds.
[0050] As specific examples of the water-soluble polymer,
polythiophene sulfonic acids, polyvinyl sulfonic acids, and
polyacrylamide sulfonic acids can be mentioned.
[0051] As the most typical example of applicable water-soluble
electroconductive polymer, a water-dispersible polythiophene
derivative as shown in FIG. 4 (i.e., PEDOT-PSS) that is obtained by
using a polystyrene sulfonate (PSS) that serves as the
water-soluble polymer, and 3,4-ethylenedioxythiophene (EDOT) that
serves as monomers of the electroconductive polymer can be
mentioned.
[0052] Furthermore, for the water-soluble electroconductive
polymer, an aqueous dispersion of a copolymer of ethyl
3-methyl-4-pyrrolecarboxylate and butyl
3-methyl-4-pyrrolecarboxylate, which are both polypyrroles, can
also be used.
[0053] In addition, an additive for improving adhesiveness,
moisture resistance, and/or weather resistance can be included as
long as the amount thereof is minute.
[0054] With regard to a particle size of the electroconductive
polymer used herein, a dispersion of those having a size of 1 nm to
100 nm as primary particles is preferably used. If the particle
diameter is larger than 100 nm, the size of the electroconductive
polymer present in the aerogel will be large, and the transparency
will significantly be decreased, e.g., to 10% or less.
Additionally, if the particle diameter is less than 1 nm,
sufficient antistatic effects cannot be realized unless an
excessive amount of the electroconductive polymer is included.
Therefore, such a range is considered to be uneconomical.
(Catalyst)
[0055] In order to promote a hydrolysis reaction of silica in the
aqueous alkaline high-molar-ratio-silicate solution, an acid
catalyst is preferably added to the solution.
[0056] With regard to types of acid used herein, inorganic acids
such as hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric
acid, sulfurous acid, phosphoric acid, phosphorous acid,
hypophosphorous acid, chloric acid, chlorous acid, and hypochlorous
acid; acidic phosphates such as acidic aluminum phosphate, acidic
magnesium phosphate, and acidic zinc phosphate; and organic acids
such as acetic acid, propionic acid, oxalic acid, succinic acid,
citric acid, malic acid, adipic acid, and azelaic acid, among
others, can be mentioned. Although types of acids used herein are
not limited, hydrochloric acid is preferable since the resulting
silica aerogel will have sufficient strength of the gel skeleton,
and sufficient hydrophobicity.
[0057] For example, the concentration is preferably 1 to 12 N, more
preferably 6 to 12 N when the acid is hydrochloric acid.
[0058] An amount of the acid catalyst added to the reaction mixture
depends on a pH value that the reaction mixture is adjusted to.
However, when 12 N aqueous hydrochloric acid is used, 0.5% to 6.0%
thereof is preferably added, and, 1.0% to 3.0% thereof is more
preferably added, given that the weight of the hydrogel is regarded
as 100%.
[0059] A sol solution that is prepared by adding the above acid
catalyst to the aqueous high-molar-ratio-silicate solution is
converted to a gel. Gelatinization of the sol is preferably carried
out inside a closed vessel that prevents a liquid medium from
volatilizing.
[0060] When an acid is added to the aqueous
high-molar-ratio-silicate solution to carry out gelatinization, the
pH during that process is preferably 5.0 to 8.0.
[0061] A temperature for gelatinization of the sol is preferably
0.degree. C. to 100.degree. C., more preferably 20.degree. C. to
90.degree. C. under ordinary pressure. That is, gelatinization will
occur even at ordinary temperatures, but the chemical reaction of
gelatinization can be accelerated by heating the reaction
mixture.
[0062] In addition, the time for gelatinization varies with the
temperature for gelatinization, and a time required for aging
(aging time described below) carried out after gelatinization.
However, the sum of the gelatinization time and the aging time is
preferably 0.5 to 72 hours, more preferably 2 to 24 hours. By
carrying out gelatinization and aging in this manner, a wet gel
that has improved strength and rigidity of the gel wall and that
hardly shrinks during drying can be obtained.
[0063] When the sol solution is soaked into fibers of unwoven
fabric or glass wools to convert it to a gel, the sol solution that
has been adjusted to a predetermined pH may preliminarily be coated
onto the fibers by use of a dispenser or the like, and then, may be
converted into a gel.
[0064] For an even industrial purpose, in terms of sufficient pot
life of the sol solution, the following way is preferable. That is,
a sol preparation in which twice the amount of the acid that is
required to cause a desired gelatinization reaction is included,
and a sol preparation not including the acid are prepared, these
preparations are discharged separately from respective tanks, and
then, are mixed together and coated directly onto the unwoven
fabrics or glass wools.
(2) Aging Step
[0065] A temperature for aging (aging temperature) is preferably
50.degree. C. to 100.degree. C., more preferably 60.degree. C. to
80.degree. C. under ordinary pressure.
[0066] A time for aging (aging time) depends on the aging
temperature. However, the aging time is preferably 0.5 to 24 hours
in view of sufficient productivity. The aging time is more
preferably 6 to 18 hours.
[0067] Furthermore, in order to prevent elution of silica, it is
effective to carry out aging in the presence of saturated water
vapor. Furthermore, it is also effective to carry out aging in a
steamed state. For the industrial purpose, it is preferable that
aging is carried out in a tank that makes it possible to stably
maintain a high-temperature and high-humidity environment, e.g. at
85.degree. C. and at a humidity of 85%. In this embodiment, in
order to prevent drying of the outermost surface of the gel, aging
is carried out in a condition in which the surface is covered with
a film of polypropylene.
(3) Hydrophobizing Step
[0068] An aged wet gel (also called hydrogel; a gel containing
water) is reacted with a silylating agent to achieve hydrophobizing
of the gel.
[0069] In this embodiment, for a hydrophobization reaction that is
provided in the method for producing an aerogel, for example, a
trimethylsilylation reaction of a silanol is carried out by use of
hexamethyldisiloxane (hereinafter, referred to as HMDSO), e.g., in
a mixture solvent of HMDSO, HCl and IPA.
[0070] In the hydrophobization reaction, HCl can also be included
at a molar ratio of 0.01 to 2.0 with respect to the amount of HMDSO
to produce TMCS which severs as an active species in the reaction
system. In that case, the concentration of the aqueous hydrochloric
acid is preferably 1 to 12 N, more preferably 6 to 12 N.
[0071] The amount of the silylating agent added to the reaction is
preferably 100% to 800%, more preferably 100% to 300% with respect
to the volume of pores in the hydrogel since hydrophobization can
efficiently be carried out. In addition, the amount of HMDSO
(silylating agent) added to the reaction mixture is determined
based on the volume of pores in the hydrogel, and, for example, in
cases where the amount of the silylating agent is 150%, this means
that 1.5 times the amount of the silylating agent to the volume of
pores in hydrogel is added thereto.
[0072] The hydrophobization reaction may be carried out in a
solvent, as needed, and, is generally carried out at 10.degree. C.
to 100.degree. C., preferably 40.degree. C. to 70.degree. C., which
are considered as temperature ranges that make it possible for the
reaction to efficiently proceed while making it possible to prevent
vaporization of the liquid.
[0073] For the solvent used herein, alcohols such as methanol,
ethanol, and 2-propanol; ketones such as acetone, and
methylethylketone; and linear aliphatic hydrocarbons such as
pentane, hexane, and pentane are preferable. Furthermore, in order
to cause the reaction to more efficiently proceed, the aged
hydrogel may be soaked in HCl in advance, and then, may be soaked
in a bath filled with a silylating agent such as HMDSO, thereby
carrying out a trimethylsilylation reaction of silanols. In order
to enhance the permeability, an amphiphilic liquid such as IPA may
be added to the reaction mixture as needed.
(4) Drying Step
[0074] In the drying step, in order to volatilize the liquid medium
in the hydrophobized gel obtained in the former step, any drying
technique can be used. For example, any known techniques such as
the supercritical drying method or non-supercritical drying method
(ordinary-pressure drying methods, freeze-drying methods, etc.) can
be adopted, and the method used herein is not limited.
[0075] However, the supercritical drying method under ordinary
pressure is preferably used in view of sufficient productivity,
safeness and economic efficiency. The drying temperature and the
drying time are not limited. However, if the gel is drastically
heated, bumping of the solvent in the wet gel may occur, possibly
causing large cracks in the silica aerogel.
[0076] If cracks appear in the silica aerogel, heat transfer may be
caused due to convection of the air, and, consequently,
heat-insulation properties may be impaired, or the silica aerogel
may be formed into a powder, thus significantly impairing easiness
in handling, although it depends on sizes of cracks. Furthermore,
if the silica aerogel is dried in a high-temperature environment,
e.g., at 400.degree. C. or more, the silylating agent, which has
maintained hydrophobicity of the aerogel, may be released through
heat decomposition, and the resulting gel may be a hydrogel that
loses hydrophobicity. Therefore, in order to suppress occurrence of
cracks, in the drying step, the gel is preferably dried at a
temperature that is sufficient to volatilize the liquid in the gel
at ordinary pressures, e.g., at 0.degree. C. to 200.degree. C., for
0.5 to 5 hours.
<Effects>
[0077] The silica aerogel obtained in this way according to the
present embodiment has sufficient electrical insulation properties
while having less incidence of charging and powder falling than
conventional aerogels. The aerogel synthesized in this way has a
pore diameter of 10 to 68 nm, which is smaller than the mean free
path of the air, and have excellent heat-insulation performance.
Therefore, the aerogel can preferably be available for use in home
electric appliances, automobile parts, the field of architecture,
industrial facilities, etc.
EXAMPLES
[0078] Hereinafter, present embodiments will be described on the
basis of examples. However, present embodiments are not limited to
the examples described below. All reactions were carried out under
the atmosphere.
<Evaluations>
[0079] For analysis and evaluation on microstructures of aerogels,
the nitrogen adsorption method called BET measurement was used, and
a fully-automatic gas adsorption amount measurement apparatus
Autosorb-3b (YUASA IONICS CO., LTD.) was used. For measurement of
heat conductivities, a heat flow meter HFM 436 Lambda (NETZSCH
GROUP) was used.
[0080] For an electric conductivity meter, a resistivity meter
Hiresta-UX MCP-HT800 (MITSUBISHI CHEMICAL ANALYTECH CO., LTD.) was
used to measure insulation resistance values of prepared sheet-like
samples.
[0081] A hazemeter HAZEMETER TC-H3DPK/3 (TOKYO DENSHOKU CO., LTD.)
was used to measure transmissivities, which serves as indexes for
representing transparency, with respect to sheets with a thickness
of 1 mm.
[0082] Furthermore, a method for confirming charging states of
prepared silica aerogel will be described with reference to FIG. 5.
Silica aerogel beads 504 were placed in a glass screw tube 501, and
the screw tube 501 was sealed with a plastic cap 502. Then, the
screw tube 501 was shaken, and a degree of adhesion of xerogel fine
powder onto the surface of glass wall in a fine-gel-powder-adhering
observation area 503 due to static electric charge was then
visually observed to compare the degrees of adhesion among
samples.
[0083] Details on conditions in respective examples and comparative
examples will described below. In addition, the conditions,
observed properties, and judgement of acceptance in the examples
and the comparative examples are summarized in Table 1. In
addition, with regard to a priority list for judgement, results of
heat conductivity measurement, degrees of adhesion of gel onto
screw tubes, and insulation resistivities were given priority in
this order.
TABLE-US-00001 TABLE 1 Evaluation Heat on charging Digit Mean
conduc- properties number of Specific pore Synthesis Conditions
tivity (adhesion insulation Thick- surface distri- Trans- Silica
Conductive (mW/ onto screw resistivities ness area bution missivity
Overall Concentration material mK) tubes) (.OMEGA.) (mm)
(m.sup.2/g) (nm) (%) Evaluation Example 1 Silica 16% SEPLEGYDA 1%
20 Acceptable 10.sup.9 1.1 290 59 32 Acceptable Example 2 Silica
16% SEPLEGYDA 0.6% 20 Acceptable 10.sup.9 1.1 312 59 36 Acceptable
Example 3 Silica 8% SEPLEGYDA 1% 18 Acceptable 10.sup.9 1 742 7.5
30 Acceptable Comparative Silica 16% None 20 Unacceptable 10.sup.12
1.1 385 37 40 Unacceptable Example 1 Comparative Silica 8% None 18
Unacceptable 10.sup.12 1 700 20 38 Unacceptable Example 2
Comparative Silica 8% Toluene-dispersed 32 Acceptable 10.sup.9 1
150 130 11 Unacceptable Example 3 polyaniline 1%
[0084] With regard to heat conductivities, 26 mW/mK, which is the
heat conductivity of still air, was used as a standard, and, when a
sample exhibited a value larger than this standard, it was
considered that synthesis of aerogel succeeded.
[0085] Specific acceptability criteria were not provided for
transmissivities and specific surface areas. With regard to mean
pore diameters, 68 nm or less (the mean free path of the air was 68
nm) was used as an acceptance criterion. This is because, when the
mean pore diameter is larger than this criterion, it is considered
that the heat conductivity exceeds 26 mW/mK.
[0086] With regard to observation of adhesion of samples onto screw
tubes, the presence or absence of adhesion of white silica fine
powders to screw tubes were confirmed by visual inspection. When a
fine powder was adhered to a screw tube, it was considered that
there was static electricity-caused adhesion. Furthermore, with
regard to insulation resistivities, a range from 10.sup.6 to
10.sup.11 was adopted as an acceptance criterion. When the
insulation resistivity is 10.sup.12 or larger, the insulation
properties are too high, and therefore, a state in which the gel is
likely to retain static electricity would be maintained. On the
other hand, in cases where the insulation resistivity is 10.sup.5
or less, if a powder of the gel or electroconductive polymer falls
out, it is required that the fallen powder is regarded as
conductive. Therefore, such a sample is not preferable as an
electrical insulation sheet.
Example 1
[0087] 0.08 g of hydrochloric acid (KANTO KAGAKU;
Shika-special-grade; 12N) serving as an acid catalyst was added to
5.02 g of an aqueous alkaline high-molar-ratio-silicate solution
(TOSO SANGYO Co., Ltd.; 16 wt % of SiO.sub.z and 0.57 wt % of
Na.sub.2O) that had been prepared from an aqueous alkaline
low-molar-ratio-silicate solution, 0.05 g of SEPLEGYDA AS-Q009
(manufactured by SHIN-ETSU POLYMER CO., LTD.) serving as an
electroconductive polymer was further added thereto, the resulting
mixture was stirred thoroughly, and the pH of the aqueous
high-molar-ratio-silicate solution was adjusted to 7.3. The sol
solution was converted to a gel at room temperature for 5 minutes,
and the resulting gel was subjected to aging in a furnace at
80.degree. C. for 12 hours.
[0088] In addition, SEPLEGYDA AS-Q009 contained 0.5 to 2 wt % of a
polythiophene resin mixture that served as an electroconductive
polymer. SEPLEGYDA refers to a polythiophene electroconductive
polymer that have transparency superior to other electroconductive
polymers, and is a trademark of SHIN-ETSU POLYMER CO., LTD. AS-Q009
refers to a solution in which the electroconductive polymer with a
size between 1 to 100 nm and trace amounts of other additives are
dispersed.
[0089] Next, hexamethyldisiloxane (hereinafter, referred to as
HMDSO; MW: 162.38; bp: 101.degree. C.; d0.764 g/ml (20.degree. C.);
SHIN-ETSU CHEMICAL CO., LTD.; KF-96L-0.65cs), HCl, and 2-propanol
were added to the aged gel. The amount of HMDS added thereto was
equivalent to 750% of 4.2 mL, which corresponded to the volume of
pores in the hydrogel (i.e., 31.5 mL; 24.1 g; 148 mmol). The
amounts of HCl and 2-propanol added thereto were 2 equivalents (296
mmol) and 1 equivalent (148 mmol), respectively, with respect to
HMDSO in terms of molar ratios. Then, the mixture was subjected to
hydrophobization in a furnace at 55.degree. C. for 12 hours in the
same manner. Two phases were recognized in the reaction solution
(upper layer: HMDSO; and lower layer: aqueous HCl), and the gel was
present in the bottom part of the lower layer at an early phase of
the reaction. However, the gel floated to the upper layer after
completion of the reaction. Then, the gel was harvested, and was
subjected to heat-drying at 150.degree. C. in the air for 2 hours,
thereby obtaining a colorless and transparent silica aerogel.
[0090] Moreover, the produced silica aerogel was placed inside the
screw tube, and the screw tube was shaken to confirm a charging
state of the sample. As a result, it was observed that adhesion of
the fine powder was suppressed, and it was confirmed that
charging-preventing effects were obtained.
[0091] Furthermore, a sol solution that had been prepared to have
the same liquid composition was soaked into a polyester unwoven
fabric 1 mm thick, and then, was converted into a gel therein. In
the same manner as the above synthesis of only the gel, the sample
was subjected to aging, hydrophobizing and drying steps to thereby
prepare a silica aerogel synthesis sheet. The prepared sheet was
evaluated. Results are shown in Table 1.
Example 21
[0092] 0.03 g of SEPLEGYDA AS-Q009 (manufactured by SHIN-ETSU
POLYMER CO., LTD.) serving as an electroconductive polymer was
added to a reaction mixture. Other conditions were the same as
Example 1.
Example 31
[0093] 5.01 g of water glass (FUJI KAGAKU CORP.; SiO.sub.2; 8.0 wt
%; 2Na.sub.2O.3SiO.sub.2.mH.sub.2O) was stirred together with 10 g
of an H-type ion-exchange resin (SUMIKA CHEMTEX CO., LTD.; Duolite
C20) until the pH exhibited 2.0. Then, the ion-exchange resin was
separated from the aqueous solution by filtration, 0.05 g of
SEPLEGYDAAS-Q009 (manufactured by SHIN-ETSU POLYMER CO., LTD.) was
added to the aqueous solution, the aqueous solution was uniformly
stirred, and then, 0.5 mol of aqueous ammonia (KANTO KAGAKU;
special-grade; 1 mol/L) was added to the aqueous solution to adjust
the pH to 4.0.
[0094] The aqueous solution was subjected to gelatinization at room
temperature for 20 minutes, and then, the gel was aged at
50.degree. C. for 24 hours. Next, HMDSO (MW: 162.38; bp:
101.degree. C.; d0.764 g/ml (20.degree. C.); SHIN-ETSU CHEMICAL
CO., LTD.; KF-96L-0.65 cs). HCl, and 2-propanol were added to the
aged gel. The amount of HMDS added thereto was equivalent to 600%
of 4.37 mL, which corresponded to the volume of pores in the
hydrogel (i.e., 26 mL; 34.0 g; 210 mmol). The amounts of HCl and
2-propanol added thereto were 2 equivalents (420 mmol) and 1
equivalent (210 mmol), respectively, with respect to HMDSO in terms
of molar ratios. Then, the mixture was subjected to
hydrophobization in a furnace at 55.degree. C. for 12 hours in the
same manner. Two phases were recognized in the reaction solution
(upper layer: HMDSO; and lower layer: aqueous HCl), and the gel was
present in the bottom part of the lower layer at an early phase of
the reaction. However, the gel floated to the upper layer after
completion of the reaction. Then, the gel was harvested, and was
subjected to heat-drying at 150.degree. C. in the air for 2 hours,
thereby obtaining 0.33 g of a colorless and transparent silica
aerogel. The mean pore diameter and the specific surface area of
the resulting aerogel were 7.5 nm and 747 m.sup.2/g,
respectively.
Comparative Example 1
[0095] In contrast to Example 1, any electroconductive polymer was
not added to the reaction mixture in the blending step. Other
conditions were the same as those in Example 1.
Comparative Example 2
[0096] In contrast to Example 3, any electroconductive polymer was
not added to the reaction mixture in the blending step. Other
conditions were the same as those in Example 3. The mean pore
diameter and the surface specific area of the resulting aerogel
were 20 nm and 700 m.sup.2/g, respectively.
Comparative Example 3
[0097] In contrast to Example 3, instead of the electroconductive
polymer SEPLEGYDA, 0.05 g of a toluene-dispersed polyaniline type
(T) (KAKENSANGYOU CORPORATION) was added to the reaction mixture in
the blending step. Other conditions were the same as those in
Example 3. The mean pore diameter and the surface specific area of
the resulting aerogel were 130 nm and 150 m.sup.2/g,
respectively.
DISCUSSION
[0098] As seen from Table 1, charging-preventing effects were
confirmed in Examples 1, 2 and 3 in which electroconductive
polymers were included, and also, it was confirmed that their
insulation resistivities were lowered to the scale of 10.sup.9.
[0099] When an aerogel is synthesized from a silica sol solution
that includes a water-glass-type material, by use of a sol-gel
reaction, it is important to synthesize a structurally-homogenous
gel from the sol preparation solution.
[0100] Judging from results of Examples 1, 2 and 3, even when
electroconductive polymers were added to the systems of aqueous
water-glass-based sol solutions, the electroconductive polymers did
not promote crystallization of silica, and the solutions came into
states of bluish transparent sol solutions (originally, colorless
transparent solutions), and it was estimated that the
electroconductive polymers were trapped inside skeletons of gels,
since the amounts of the electroconductive polymers added to the
solutions were very small, i.e., 1% or less. This was obvious
because any bluish ingredients were not eluted in the processes of
hydrophobization, and it was considered that the electroconductive
polymers were solidified together with silica particles in the
aerogels. From these observations, it is considered that
polypyrrole-type, polyaniline-type, and polythiophene-type
electroconductive polymers can bring about the same effects as long
as these materials have water-soluble side chains.
[0101] Furthermore, among a number of electroconductive polymers,
when electroconductive polymers that do not have, in their
structure, any functional groups compatible with water and that are
dispersible only in organic solvents or the like were added to
systems of aqueous water-glass-based sol solutions, the solutions
immediately yielded a white turbidity. Thus, it was revealed that
such electroconductive polymers inhibit gelatinization, and
therefore, such electroconductive polymers were considered
unsuitable at a step of preliminary studies.
[0102] Meanwhile, in consideration of results of Comparative
Example 3, even if the same types of electroconductive polymers are
used, crystallization of water-glass-based sol solution will occur
in dispersions using organic solvents such as toluene, or in
systems including alcohols. Therefore, it is considered that a
homogenous gel synthesis reaction through the sol-gel reaction of
silica will be impaired in such dispersions or systems. As a
result, even if insulation resitivities are reduced as a
consequence, it is obvious that, according to such dispersions or
systems, characteristics such as low heat conductivity and
excellent transparency that silica xerogels naturally possess
cannot be retained. In present embodiments, for example, the
molecule that has a structure including
poly(3-thiophene-ethylsulfonic acid) that is obtained by
introducing a substituent group into a monomer and that realizes
water solubility, as shown in FIG. 3 can be adopted.
[0103] In view of the above observations, water-soluble
electroconductive polymers that intramolecularly include a sulfo
group compatible with water that serve as a dopant/water-dispersing
agent are preferably used. For example, polythiophene sulfonic
acids; polyvinyl sulfonic acids; polyacrylamide sulfonic acids; a
water dispersible polythiophene derivative (PEDOT-PSS) using a
polystyrene sulfonate (PSS), which serves as the water-soluble
polymer, 3,4-ethylenedioxythiophene (EDOT), which serves as a
monomer of the electroconductive; and an aqueous dispersion of a
copolymer of ethyl 3-methyl-4-pyrrolecarboxylate and butyl
3-methyl-4-pyrrolecarboxylate, which are both polypyrroles, are
preferable. In present embodiments, these materials are
particularly effective in realizing charging-preventing effects
while maintaining heat-insulation performance that aerogels
intrinsically possess.
[0104] In addition, aerogels according to embodiments can be
utilized as heat-insulation materials in combination with
substrates such as fibers. FIG. 6 shows a cross-section view of a
heat-insulation material 106. The above aerogel 105 according to
the above embodiment is located between fibers 107. The aerogel 105
that is in a sol state may be caused to penetrate into the fibers
107, or the fibers 107 may be soaked in the aerogel 105 that is in
a sol state, thereby preparing the heat-insulation material
106.
[0105] Silica aerogels according to the disclosure can be utilized
as heat-insulation materials, and the heat-insulation materials can
preferably be available for use in home electric appliances,
automobile parts, the field of architecture, industrial facilities,
etc.
* * * * *